Interfacial pH gradients suppress HER at high currents in zinc metal batteries

Aqueous zinc metal batteries (AZMBs) are attractive alternatives to Li/Na systems due to their abundance, safety, low cost, and high capacity. Unfortunately, cycling efficiency is hindered by hydrogen evolution reaction (HER) at the anode. Paradoxically, higher current densities often yield improved Coulombic efficiency (CE), defying classical electrochemical expectations. Here, we resolve this paradox by revealing the role of interfacial pH gradients in zinc electrodeposition. Through advanced measurements, including electrochemical mass spectrometry and fluorescence microscopy, we show that steep pH gradients emerge at high currents and low capacities, fostering rapid formation of a uniform solid electrolyte interphase (SEI) that suppresses hydrogen evolution and enhances CE. This advantage diminishes at larger capacities or extreme currents due to convective instabilities. We present a unified framework, integrating pH gradients, SEI formation, HER suppression, zinc nucleation and growth, and convection, and we propose charge-discharge protocols that extend cycle life at practical and real capacities, advancing commercially relevant AZMBs.